1. Field of the Invention
The present invention relates to detection devices and image forming apparatuses including such detection devices.
2. Description of the Related Art
In an image forming apparatus that forms an image by causing toner to adhere electrostatically to a photosensitive member, the density of the formed image will change if a charge amount of toner (called a “toner charge amount” hereinafter) changes due to temperature, humidity, or the like. In other words, more toner will adhere to the photosensitive member as the toner charge amount drops, and thus an image having a higher density than a desired density will be formed. On the other hand, less toner will adhere to the photosensitive member as the toner charge amount rises, and thus an image having a lower density than the desired density will be formed.
Accordingly, a method is known that controls image forming conditions such as an exposure light amount, a developing bias, and a charging potential for forming an electrostatic latent image on the photosensitive member based on a result of measuring the toner charge amount, in order to control the density of an image.
In U.S. Pat. No. 5,006,897, a probe including a piezoelectric crystal resonator (an oscillator) is caused to attract toner from a magnetic brush roller, and the toner charge amount is then calculated based on a mass calculated from a change in the frequency of the piezoelectric crystal resonator and a change in an amount of electric charge on the magnetic brush roller.
However, there is a problem in that when the probe is caused to attract the charged toner, an excessive voltage is applied to the oscillator that configures the probe, an oscillation circuit that drives the oscillator, and so on. As a result, the oscillator, the oscillation circuit, or the like will be damaged, electrodes will separate, and so on, and the toner charge amount cannot be measured.